As others said -- do you mean "Fairfax City" or all of Fairfax County? (By the way, welcome!!) The county is huge and there would be a ton of difference in commute time depending on where you live!
If you are in Fairfax City, you could try Vienna Pediatrics, part of the Capital Area Pediatrics group, on Maple Ave. (that's Route 123) in downtown Vienna. We use them and have had good doctors there (Sawyer, Morris and MacAndrew in particular). They have Saturday hours and even do Sunday morning appointments if needed, and they have a "nurses' triage line" so you can have a real discussion with a nurse to decide whether your child's issue is one that warrants a visit. (The triage line has even helped me out big-time when we were abroad and I had a question and no pediatricians were available where we were!).